The present invention relates to an apparatus for analysing particle dimensions of particles contained in a fluid.
Known apparatus for analysing particle dimensions of particles contained in a fluid use a focussed light beam to sweep the particle to be measured. The time is then measured in which the respective particle interrupts the light beam during transmissive measurement or reflects the light beam during reflective measurement. The determination of the particle dimensions from the above-mentioned time is possible only if the relative speed of particles in relation to the light beam is known precisely. For this purpose, the focal point is displaced along a circular path or a zig-zag path at a speed which is great compared with the speed of the individual particles.
In the case of large particle concentrations it is necessary to bring the focus sufficiently close to the fluid-side window that the extinction of the light beam in the fluid remains small. On the other hand the focal point must maintain a minimum distance from the fluid-side window which is greater than the radius of the greatest particles to be measured. Otherwise the particle would not be determined in its full size.
Such particle measuring apparatus is disclosed in U.S. Pat. No. 3,858,851, in which the scanning laser beam is displaced on a circular path by incorporating a plane-parallel plate obliquely into the beam path and rotating it about an axis running parallel with the direction of the incident laser beam. This generates a parallel displacement of the laser beam and hence the desired path. The focal point of the device lies within the fluid and does not have to be set separately, because the fluid is intrinsically transparent.
In the particle measuring apparatus disclosed in GB-A-2243681 the incident laser beam falls through a rotating prism, and the circular path of the focal point is generated in this way. Particles are analysed on a specimen slide with this measuring instrument. In addition the particles have to lie in the area of the focal point of the optical device. The holder of the specimen slide can be displaced for this purpose.
Another apparatus for reflective operation is known from EP-A-289200. With this apparatus, which can be immersed in a fluid as a probe, a focal point rotating along a circular path is likewise generated. For the setting of the distance between the fluid-side window and the focal point a separate focussing device is provided, which in the course of operation sets the reflected signal to a maximum achievable amplitude level as a function of the properties of the fluid. However, the difficulty arises in practice that the focussing device necessitates certain structural measures and in addition has to be activated by the process control system. The focussing device therefore leads to a complicated layout of the apparatus and to a further measurement and control variable which has to be monitored or generated.